The objective of this contract is to support a variety of projects aimed at identifying and characterizing the correlates or markers of the immune status to infection with HTLV-III/LAV and to the subsequent progression of the infection to AIDS. The observation that some members of risk groups do not become infected (seroconvert) following a very high probability of exposure to the virus, and the finding that most individuals infected with the virus have not progressed to symptomatic disease let alone AIDS lends support to the view that protective immune mechanisms exist. A definitive understanding of the immune mechanisms that either prevent an initial infection or the progression of an infection to the fully developed disease would help provide a firmer scientific basis for vaccine development activities.